Erythemal meter



H. S. SCOTT ERYTI'iEMAL METER Nov. 21, 1950 2 Sheets-Sheet 1 Filed Dec.24, 1947 Wave/eryzh 6422751772775) lnvervtor':

Hoyt S. Sco t't,

Patented Nov. 21, 1950 UNITED OFFICE Hoyt S. scotfi l i e liidilfifiiignor to Geneial Electric York '(flompany, a corporation of- Newp li ation De ember 24, less ning 12w;

.r ebie mete "9 I se m, .gol. 250-:83)

this type is available. Ra ation in the cry h e. ee s wh ele pa es neare by artificial sources is usually mixed with radit n. o Q h r. waleng hs. sashes visib e i h infrared near u tr -Yield rad t on anradiation shorter than 2800 A. wave length. L g n it ve t ll o the. b 9sia ba rie ayer type, which a ompa t an misses... are sensitive tomuphoi this radiation so that they, ann e ed at s t y b direct x sur to suchmixed radiations measure only the radiation in theerythemalspectral-region.

The principal object of my invention is to provide a meter comprisingcells'of the'blocking layer-- type for measuringradiant energy in thecrythemal spectral region. of 2800 to-3200- A. wave! length, whenpresent either alone-or 'l'nixed-with radiant energy of otherwavelengths.

Further objects and: advantages-oi the invention will appear from thefollowing description and from the. drawing in which Fig. l is aperspective view of a meter mbodying. my invenr. tion;-Eig. 2 is -a*'dia-r.am matic showing, of the system of radiation filters and a lightsensitive cell used-in-the light meter of Fig. 1-; Fig 3, isa chartshowing the characteristics of a group offiltersused in, the meter; Fig.4 is anot r chart. showing the characteristics. of another 'g'roupoffilters; Fig. 5 is a chart showingtheerythemal urve; and Fig. 6- is. .a.wiring d figram, of: the meter Qf-IiigS. 1 and 2.

Referring to Fig. 1 of the drawing the light meter comprises. a currentmeasuring devicev l--, such as a microammeter. and rectangular casing 2containing four light sensitive cells 3-of the barrier or blocking layertype. A cover 4 having. a to including a number of filters, forabsorbing radiant energy of various wavelengths, as de-.v scribed below,is provided for the casing; 4?. cells 3 are electrically connectedtogether in. a series parallel circuit across the terminals of: themicroammeter. I, as shown in Fig. 6, t obtain theoptimum energy transferfor'measurement by the microammeter. The latter isv calibrated. so.

2 toj be. readable directly in E-yitons per square eet; aaErviiaa-ii negleaned as. sash an. amount.

ta iaai'il lx wil a 8 the. same apaeitr o.

ree ee e rth ia deni rate-redd nin f t e aatanaedhu aaa s m ere at s.radiant energy at 2967 A. wavelength.

fhel sh isensiii e s ll 3 a e 9 a ype cemea a lx a ailab e aeea erespansi e t a ian nsi a ee-rin ave en th between a o .8 .0; nd ane-minihen-ne r sponse in. a. band; 05: ma-vis hle sestmm be ween a out. 9 to..000 AI ccordin ly the op. 5. of; cover 4- also ludes-a. nhpsnherdescribedher inaiteror cona n er an l armine h r diat ons of between.-

280 (t 3309 wavelength to. be measured. o.

radiati 11 Off between to. 6.000 A. wavelength.

a hieh -the cells-.3 are mo t re pon whefirstfilter G. of he s tem. inthe top. 5- ct 4 is known commerciallyas red purple "Gamin #aflfia andisv available under. this. (165?. ignation from the Corning Glass Works,Corning,

New. York- It; isag ass filter abou 3 mm. thic S.pO1.i hed On bothsides, and its transmission. curve is shown. in Fig. 3 or the drawing.As shown in his fieure and in Fi fil er 6 tran mits; ultra=.violetradiation and absorbs the radi: ationabout $100; A. Its peaktransmission is he: tween .29 00 andZ'IQOA. and it transmits between'75; and 8.5 per cent ofthe incident radiation. in the. erythemal region2800 to 3200 A. It also transmits a slight amount of infrared radiationwhich must be absorbed by other filters.

The second filter 1' is Gorex D Corning #9700 which absorbs; radiationbelow 2500 A. trans: mitted by filter- 6 andtransmits an extremely smallamount at 2531?: A. and approximately 45 to-% of the incident radiationbetween 2800:

amt-3200 A. Italso. transmits the small amount" ot infrareda mentionedabove. This-filter 8 is also a glass filter} about 3 mm. thick, ispolished. on both sides, and is. commercially.- available fromtheBorning Glass Works, Gaming. New York.

The twofilters i and 'l', which I- designate asthepre-filter: group,thusv cut ofior absorb sub-.- stantiallyall'theiricident radiationabove. 4200A. and below. about. 2500A. and. transmit energ be.- tweenthe. of 2500. A. and-4100. A. Thegreater part of theenerg-y: between2800 A. and 3200 A. is transmitted thereby and also a small amount-aslow as. 2500 A. andasmallamount ofi infrared.

The phosphor mentioned: above also blocks out visible light fromthesourceif any gets through thefilters 6- and-'1, as may-happenwithhigh in.-

tensity. Sources. The phosphor-- converts the:

3 erythemal radiation between 2800 to 3200 A. to visible light inproportion to the amount of erythemal radiation incident thereon. It isindicated at 8 in Fig. 2 of the drawing and consists of fluorescentmagnesium tungstate (MgWO4) powder applied as a coating by known method,such as by using a suspension of the powder in a suitable binder, suchas butylacetate to the surface of the .glass :filte'r l facing away fromthe light source and toward photoelectric cell. I call this phosphorcoating the exciter since it emits visible light which excites the lightsensitive surfaceof the photoelectric cell and causes it to generate anelectric current. As shown by the sensitivity curve of the exciter inFig. 3, the exciter 8 is most sensitive or responsive to radiations ofabout 2850 A. It acts as a filter to some A. which is within the band atwhich the cells 3 are most sensitive, that is, between 4000 A. and 6000A. It is this energy only which impinges on the cells 3 and its curve isshown in Fig. 4.

The post-filter group thus absorbs all radiation outside the spectralband to which the cells show their maximum response. I call theradiation outside said band stray radiation in the claims and intend todesignate thereby the radiation outside the band transmitted by theprefilter group and the exciter and also that genextent for radiation oneach 'side of 2850 A. The

exciter 8 is not responsive to radiations above 3400 A. transmitted bythe filters 6 and l and,

while its sensitivity to radiations of 2900,.Znf-is transmission curvesof the' two filters and the mg has.-begn takem The micma eter is ansensitivity curve of the exciter and represents" egtreme'iysgnsifiveone; though rugged enough the eii'ective energy incident-on theexciter."

Fig. 4 of the drawing shows the characteristicsof a group of filters Idesignate as the postfilter group since it filters the energytransmitted and emitted by the exciter 8. These filters are necessaryfor absorbing the small amounts of infrared and energy below 3700 A.transmitted by the filters 6 and 1 and the exciter 8, because the cells3 are responsive to energy in these regions.

This chart also shows the light emission curve of the exciter 8 whichreaches a maximum at a point between 4700 and 5100 A.

The first element 9 (Figs. 2 and 4) of the postfilter group consists ofa thin film of methyl methacrylate which is applied as a protective filmover the phosphor coating constituting the exciter 8 and which transmitsthe visible light erated by the exciter.

The cells 3 are of a well-known type and are made up of .an iron supportstrip coated on its 7 exposedjsurface with a layer of selenium and athin, li'ght pervious coating of silver over the selenium coating. Athin, light transmitting lacquerrfilmis applied to the cell to protectthe -flhefseries parallel circuit including the cells I '3 rand themicrofainmeter I is shown in Fig. 6 of emitted by the exciter 8. It alsotransmits the undesirable infrared radiations and the ultraviolet energybelow 2800 A.

The second filter I 0 (Figs. 2 and 4) in the postfilter group is aplastic filter known as Chromex #2324 and is available under thisdesignation from Dufax-Chromes Limited of London, England. This filterabsorbs energy below 3700 A. but transmits most of thevisible lightemitted by the exciter 8. It also transmits infrared radiations.

The third filter ll (Figs. 2 and 4) of the postfilter group is a glassfilter approximately 3 mm. thick, polished on both sides and knowncommercially as Corning #4010 sextant green. This transmits the visiblelight. in the limited'range of from 4300 to 6300 A. and absorbs thesmall amount of residual energy in the infrared which would energize thecell slightly. It also absorbs most of the long wavelength ultra-violet,but if used alone without the filter I0 I have observed that ittransmits enough of such long wavelength ultra-violet to affect theaccuracy of the erythemal meter. Thus, both filters l0 and H arenecessary for the best results. The

maximum transmission of filter.ll..is at 5300 thefdraw'ing. This circuitgives the maximum energy transie from the cells 3 to the mioroammeter-g:The microamnieter is provided with two push buttons, !2 anqtzfi; one ofwhich, l2 .connects the microamnie'terfacross the cells when it ispushed: downward-rand the other of which, I 3, brings the pdinteifllquickly to zero after a readfor transportation'and'handling andaccurately measures currentjqfjfr om one fifty millionth of an ampereuU-tobnelxhicroampere. Microammeters of this. type are available fromthe Hickok Electrical Instrument. Qompany of Cleveland, OhlO. s 1

In ordert at theerytherhal meter may be used for the purpose:contemplated its responses should correspond closely: to the'erythemaleffectiveness curve. As pointed 'out-abovethe erythemal band response isat approximately 3000 A as shown by the second curve of Fig. 5. Thus,the meter may be read directly to ascertain the erythemal component ofthe radiant flux in solaria.

Solaria energy generators have components in addition to erythemalultra-violet, such as visible light, infrared and near ultra-violet. Theerytliemal meter does not register-any Of this energy outside theerythemal region The safe exposure me of the human skin. to theerythemal radiation, measured in minutes required for minimumperceptible erythenia, may be indicated on the dial of the microammeteras shown at 15 in Fig. 1 of the drawing.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is: l. 'A meter for measuring erythemal radiation comprising alight sensitive cell having its maximum response in a band of thevisible spectral erythemal radiation, means for converting thetransmitted erythemal radiation into radiation of longer wave lengthincluding visible light in the spectral band in which the cell has itsmaximum response and means including a Corning 1010 filter between saidsecond named means' and said cell for absorbingall stray radiation,including infrared radiation, whereby the meter response correspondsclosely to the erythemal efiectiveness curve.

.2. A meter for measuring erythemal radiation comprising a lightsensitive cell having its maximum response in a band of the visiblespectral region and being responsive also to radiation outside saidband, a current measuring instrument connected-to said cell and a coverover the light band and a current measuring instrument for includingvisible light in the spectralband in which the cell as its maximumresponse and means including Chromex #2324 filter and a Corning #4010filter between said powder and said cell for absorbing all strayradiation, including infrared radiation, whereby the meter resensitivesurface of the cell comprisingifa ,red

purple Coming #9863 first filter andaCorexD Corning #9700 second filterfor transmitting; the

erythemal radiation and absorbing substantially. all other radiation, acoating of fluorescentama nes ium tungstate powder on thesurfabejbf-isaidf #9700 filter facing away fIOll'ig'fSflidfirst-filten-a'. thin film of methyl methacrylate covering said--powder, a Chromex #2324 filfterzin the-;patl r-;of

, light emitted by said powder, a; Corning 1010 sextant green filtermountedil, ithe''path of the sponse corresponds closely to the erythemaletfectiveness'curve.

5. A meter for measuring erythemal radiation light transmitted bysaid-Chromexffi-lter anda light sensitive cell having response in thespectral band of the'lighttransrnitted by said Corning #4010 filterwhereby the-meter response corresponds closely-.to"the*erythemaleffectiveness curve.

3. A meter for measuring erythemal-radiation comprising a plurality'of-llght-sensitive cells of the barrier layer type havingtheir-maximumresponse in a band of the visible'fspectrum and being responsive also toradiation outside said band and a current measuring instrument for saidcells, the latter being'nonnectied in a series parallel circuit acrosssaid-instrument and a cover over the light sensitive surface ofthecellscomprising means for absorbingsnbsjtantially all radiation other thanerythemalifrf'adiation, means for converting the transmitted erythemalradiacorresponds closely to the erythemal effectiveness curve.

4. A meter for measuring erythemal radiation comngising a plurality oflight sensitive cells of the barrier layer type having their maximumresponse in a, band of the visible spectrum and being responsive also toradiation outside said comprising a plurality of light sensitive cellsof thebarrier layer type having their maximum respouse in a. band of thevisible-spectrum and being1'responsive1also to radiation outside saidband. and a'current measuring instrument for said cells, the latterbeing connected in a series parallelcircuit acrOSssaid instrument whichis equipped with a dial calibrated in minutes required for minimumperceptible erythema for directreading, and a cover over the lightsensitive surface of the cells comprising means for abing substantiallyall radiation other than erythemal radiation, a fluorescent magnesium .1g -tungstate powder rfor converting the transmitted erythemal radiationinto radiation of longer wave length including visible light in thespectral band in which the cell has its maximum response and meansconsisting of a thin film of methyl methacrylate coverin the saidpowder, a Chromex #2324 filter and a Coming #4010 filter be- :tween saidpowder and said cell for absorbing all stray radiation, includinginfrared radiation, whereby the meter response corresponds closely tothe erythemal effectiveness curve.

' HOYT S. SCOTT.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,114,163 Bird Apr. 12, 19382,128,110 Wilson Aug. 23, 1938 2,200,853 Porter et al May 14, 19402,349,754 Porter May 23, 1944 OTHER REFERENCES Storage Batteries byVinal, 3rd edition (1940) page 5. i

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